Body coupled communications (BCC) or body-based communication has been proposed as a promising alternative to radio frequency (RF) communication as a basis for body area networks. BCC allows exchange of information between a plurality of devices which are at or in close proximity of a body of a human or an animal. This can be achieved by capacitive or galvanic coupling of low-energy electrical fields onto the body surface. Signals are conveyed over the body instead of through the air. As such, the communication is confined to an area close to the body in contrast to RF communications, where a much larger area is covered. Therefore, communication is possible between devices situated on, connected to, or placed close to the body. Moreover, since lower frequencies can be applied than typically applied in RF-based low range communications, it opens the door to low-cost and low-power implementations of body area networks (BANs) or personal area networks (PANs). Hence, the human body is exploited as a communication channel, so that communication can take place with much lower power consumption than in standard radio systems commonly used for BANs (e.g. ZigBee or Bluetooth systems). Since BCC is usually applied in close proximity to the body, it can be used to realize new and intuitive body-device interfaces based on contact or proximity. This creates possibilities for many applications in the field of identification and security.
FIG. 1 shows a schematic diagram indicating involvement of a human body in a BCC communication system. Small-sized BCC devices without direct skin contact can be realized by exploiting capacitive coupling to the human body. A two-electrode TX device generates a variable electric field that is coupled to the human body; a two-electrode RX device senses the variable electric potential of the human body with respect to the environment.
Measurements have shown that a typical body channel has a high-pass character, with a lower corner frequency determined by the input impedance of the RX device and by the capacitance of the electrodes. The signal attenuation is less than 80 dB for devices positioned at various distances on the static or moving human body. With respect to interferences, the body picks-up a significant amount of interferences in the frequency band below 1 MHz, while for higher frequencies the level of interference stays below 70 dBm and their frequency spectrum is to a great extent dependent on the environment. Hence. the established body-channel properties make the frequency band between 1-30 MHz especially attractive for BCC as this band can provide sufficient data-rate for healthcare or consumer applications (up to 10 Mb/s) and the impact of radio frequency (RF) interference is less, as the body does not act as an efficient antenna.
BCC can be technically realized by electric fields that are generated by a small body-worn tag, e.g., being integrated into a credit card or another suitable device attached to or worn in close proximity to the body. This tag capacitively or galavanicly couples a low-power signal to the body. Sometimes this body-coupled communication is referred to as “near-field intra-body communication”. BCC is a wireless technology that allows electronic devices on and near the human body to exchange digital information through capacitive or galvanic coupling via the human body itself. Information is transmitted by modulating electric fields and either capacitively or galvanicly coupling tiny currents onto the body. The body conducts the tiny signal to body mounted receivers. The environment (the air and/or earth ground) provides a return path for the transmitted signal.
FIG. 2 shows an exemplary body communication system structure, where data signals are transmitted via couplers placed near or on the body. These couplers transfer the data signal, either galvanic or capacitively, to the body. In the example of FIG. 2, one coupler or electrode provides ground potential GND and the other coupler or electrode is used for transmitting/receiving a signal S. In FIG. 1, transmission is from a transmitter (TX) 10 to a receiver (RX) 20 over an arm is depicted. Generally, every node can in principle act both as transmitter and receiver, i.e., as a transceiver (TRX), and communication can take place from everywhere on the body. Data transfer via a body channel can be used for frequencies from about 100 kHz up to about 100 MHz. Frequencies below 100 kHz can be affected by significant electrostatic interference in the body channel. At frequencies above 100 MHz the wavelength, i.e. <3 m, comes in the range of the length of (parts of) the human body. Consequently, the human body starts to act as an antenna. Consequently, there is the possibility that the BCC nodes located on different bodies can communicate which each other, using the “human body antenna”. For even higher frequencies, even the couplers start acting as antennas. Hence, communications can also take place when the (human) body is not present as communication medium. Both effects are unwanted, since only devices placed on or near the same (human) body are supposed to communicate.
Given the characteristic of the body channel and the bandwidth of interest, an interesting solution for BCC proved to be direct coupling of digital wideband signals to the human body without any kind of modulation or up-conversion. The use of wideband digital signals for BCC communication is an efficient way to provide high bit-rate with very low power consumption and simple configurability. Nevertheless, this approach requires a receiver with a bandwidth large enough to correctly receive this kind of signals and consequently open to environmental noise and interference. Receiver architectures proposed so far overcome this problem by performing a significant high-pass filtering and detecting the peaks that correspond to the signal transitions. However, such an approach suffers from the problem that the high pass filtering also attenuates the wideband wanted signal and that the receiver chain is open to interference at higher frequencies which are most likely to appear due to the specific characteristics of the body channel.